Chop leeks and rinse them well.
Heat olive oil over medium heat.
Add the leeks and cook for 5 minutes, stirring every minute.
Add the squash and broth.
Season with salt and pepper, then stir in dill.
Bring to a boil and then simmer for 20 minutes.
Remove from heat and let stand for 30 minutes.
Puree with a standard or immersion blender, then add yogurt and lemon zest, whisking well.
Transfer to a covered storage container and chill for at least 3 hours before serving.